This application is based on and incorporates herein by reference Japanese Patent Application No. 2001-27813 filed on Feb. 5, 2001.
1. Field of the Invention:
The present invention relates to a fuel injection control system for an internal combustion engine. More specifically, the invention relates to a fuel injection system for compensating fuel transportation lag, of a fuel transportation system, which transports fuel injected from a fuel injection valve to a cylinder of an internal combustion engine.
2. Related Background Art:
In many gasoline engines mounted on vehicles, a fuel injection valve is attached to an intake pipe and fuel (gasoline) is injected to an intake port. In the intake port injection of fuel, some of the fuel injected from the fuel injection valve is directly taken into a cylinder, but the rest of the fuel is adhered to the internal wall and associated parts of the intake port, and after that, the fuel gradually evaporates and is drawn into the cylinder. As an equation of modeling the behavior of the fuel in such a fuel transport system, the following Aquino equation is known:
MF(t)=(1xe2x88x92xcex94t/xcfx84)xc2x7MF(txe2x88x92xcex94t)+Xxc2x7GF(txe2x88x92xcex94t) 
where MF(t) is an amount of fuel adhered to the wall face at the present time t, xcex94t is an operation cycle, xcfx84 is a fuel vaporization time constant, MF(txe2x88x92xcex94t) is an amount of fuel adhered to the wall face at the time of operation of last time, X is a fuel adhesion rate, and GF(txe2x88x92xcex94t) is a fuel injection amount at the time of operation of last time.
In JP-A No. 8-177556, it is proposed to calculate the fuel injection amount GF(t) by the following equation by using the amount MF of the fuel adhered to the wall face calculated by the above equation. GF(t) is calculated as follows:
GF(t)=GFET/(1xe2x88x92Axcex1)xe2x88x92Axcex1xc2x7MF(txe2x88x92xcex94t) 
where GFET is a required fuel amount, and Axcex1 is obtained by sequentially multiplying an Aquino operator xcex1 (=1xe2x88x92xcex94t/xcfx84) calculated every sampling as shown by the following equation:
Axcex1=xcex1(t)xc2x7xcex1(txe2x88x92xcex94t)xc2x7xcex1(txe2x88x922xcex94t)xc2x7. . . xc2x7xcex1(txe2x88x92nxcex94t) 
In the fuel injection amount controlling method of the publication, each physical parameter such as the fuel vaporization time constant xcfx84, wall face adhesion rate X, and Axcex1 must be calculated using an arithmetic expression, a map, and/or the like. Consequently, the load on the CPU is high and the number of physical parameters to be calculated is large. This means that a number of adapting steps is required when the method is applied to an actual vehicle, and high development costs are a drawback.
At least one embodiment of the invention has been achieved in consideration of such circumstances and its object is to provide a fuel injection amount control system for an internal combustion engine. Further, the system will realize low development costs and facilitate actual adaptation of the system to a vehicle and also reduce a load on the CPU.
In order to achieve the object, according to at least one embodiment of the invention, there is provided a fuel injection amount control system for an internal combustion engine. The system compensates for a fuel transportation lag by using a fuel transportation lag model obtained by modeling a fuel transportation lag of a fuel transportation system that transports fuel injected from a fuel injection valve into a cylinder and associated intake system within an internal combustion engine. Additionally, physical parameters such as a fuel vaporization time constant, a wall face adhesion rate of an injected fuel, and the like are included in the fuel transportation lag model and converted to a small number of adaptation parameters. With such a configuration, the number of parameters to be computed is reduced, so that the number of adaptation steps for adapting the system to an actual vehicle, development costs, and the load on a CPU can all be reduced.
It is also possible to construct the adaptation parameters by a reference adaptation parameter and a correction factor, use a system identification value or a physical measurement value as the reference adaptation parameter, and correct a wall face adhesion correction amount obtained by using the reference adaptation parameter by the correction factor. For example, by adapting the correction factor in accordance with fluctuation of an air-fuel ratio, the fluctuation in the air-fuel ratio can be converged with a high response.
The fuel transportation lag model may contain a configuration such that a fuel transportation lag element A, due to adhesion of the injected fuel onto the wall face, and a first-order lag element B, for compensating a model error of the fuel transportation lag element A, are coupled in series. Fluctuations in the air-fuel ratio at the time of acceleration/deceleration are caused not only by the fuel transportation lag due to the adhesion of the injected fuel to the wall face, but also factors such as an error in measurement (estimation) of an air volume charged in a cylinder. The error in measurement (estimation) of the cylinder charging air volume can be approximated by the first-order lag of the fuel transportation lag. Consequently, by coupling the first-order lag element B to the fuel transportation lag element A in series, the model error due to the error in measurement (estimation) of the cylinder charging air volume or the like can be compensated, so that accuracy in computing the fuel correction amount can be improved.
An equation for computing a fuel correction amount by using the fuel transportation lag model may be constructed using a compensation term for the fuel transportation lag element A and a compensation term for the first-order lag element B. With the configuration, the computation equation of the fuel correction amount is simplified to two compensation terms. It further facilitates the adaptation to an actual vehicle.
In this case, in a compensation term for the fuel transportation lag, a first wall face adhesion correction amount may be obtained. This is done by multiplying a deviation between the wall face adhesion fuel amount in a steady driving mode and a wall face adhesion fuel amount at a present time, a deviation between a present intake manifold pressure and a smoothed intake manifold pressure, or a deviation between a present intake air volume and a smoothed intake air volume with a first reference adaptation parameter and a first correction factor. With the configuration, the first wall face adhesion correction amount for compensating the fuel transportation lag can be computed with a high degree of accuracy by a simple arithmetic operation.
Duration of the first wall face adhesion correction amount may be expressed by a function of the fuel vaporization time constant. Thus, the duration of the first wall face adhesion correction amount can be properly set in accordance with the evaporation characteristics of the fuel adhered on the wall face.
In a compensation term for the first-order lag element B, a second wall adhesion correction amount may be obtained in two ways. First, by multiplying a deviation between a required fuel amount of the present time and a required fuel amount of the previous time, or secondly, by multiplying a deviation between an intake manifold pressure of this time and an intake manifold pressure of last time, with a second reference adaptation parameter and a second correction factor. With the configuration, the second wall face adhesion correction amount for absorbing the model error can be accurately computed.
Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention.